This invention pertains to a type of curable silicone rubber composition. More specifically, this invention pertains to a type of curable silicone rubber composition that has excellent flame-retarding property and good electrical characteristics, as well as its manufacturing method.
Silicone rubber has excellent properties, such as heat resistance, weatherability, and electrical characteristics and is therefore used in various fields. However, the conventional silicone rubber has the disadvantage of being flammable. Consequently, various types of compounds have been proposed to develop self-extinguishable (flame-retarding) silicone rubber. For example, Japanese Kokoku Patent No. Sho 44(1969)-2591 proposed a type of composition prepared by adding hexachloroplatinic acid, platinum-olefin complex, or another platinum compound to a silicone rubber composition. However, when this type of platinum compound is used alone the effect in improving the flame-retarding property is small. Consequently, people have proposed many types of compositions prepared by combining platinum compounds with inorganic fillers so as to improve the flame-retarding property, such as the composition prepared by adding a platinum compound and fumed titanium dioxide to a silicone rubber composition (Japanese Kokoku Patent No. Sho 47(1972)-21826), the composition prepared by adding a platinum compound and fine powdery manganese carbonate to a silicone rubber composition (Japanese Kokoku Patent No. Sho 51(1976)-23979), and the composition prepared by adding a platinum compound and iron oxide to a silicone rubber composition (Japanese Kokoku Patent No. Sho 51(1976)-35501).
However, these silicone rubber compositions still fail to provide a satisfactory flame-retarding property. In addition, the aforementioned composition prepared by adding a platinum compound and fumed titanium dioxide has the disadvantage of the electrical characteristics of the silicone rubber deteriorating due to moisture. The aforementioned composition prepared by adding a platinum compound and fine powdery manganese carbonate has the disadvantage that when an acyl-type organic peroxide is used as a curing agent, curing is hampered, and the compound cannot be cured well. The aforementioned composition prepared by adding a platinum compound and iron oxide has the disadvantage that if the amount of iron oxide is not large it is hard to improve the flame-retarding property, yet a large content of iron oxide leads to deterioration in the mechanical strength of the cured silicone rubber.
It has been found that for these silicone rubber compositions, the flame-retarding property is improved when the content of the inorganic filler, a nonflammable ingredient, is increased, and it can be further improved when the content of the polyorganosiloxane, a flammable substance, is reduced. However, for the silicone rubber compositions prepared by adopting these measures, the moldability is poor, and the mechanical strength of the silicone rubber molding after heat curing decreases. Due to these disadvantages, their applications are limited.
It has been found by the present inventors that when prescribed additives are added to a prescribed type of curable silicone rubber composition the flame-retarding property of the silicone rubber is drastically increased after curing. Also, the electrical characteristics as represented by tracking resistance can be improved significantly.
Therefore, the purpose of this invention is to provide a type of curable silicone rubber composition that can form silicone rubber with excellent flame-retarding property and electrical characteristics without sacrificing mechanical strength after curing. This invention also provides a manufacturing method for this type of curable silicone rubber composition.